This is what i ended up doing, still using a 300 watt led for veg
Went with a dimmable 400 watt hps, started it out at 75%, and the fan handled it easily, so i went to 100%, with the fan at 80%, and it stays at around 78.
I got a lux meter and it says i'm getting 50k at the tops of those plants. the led puts out 24k, plenty for veg, but not close to enough for flowering.
Had the exhaust going out the top back of the flowering box, which is actually 28 x 32 x 48. this was just a little too tight, so i plugged up the hole, and ran it through the top of the veg box. I taped around all the holes so they're light tight before i actually put plants in there.
i also used the booster fan i had to blow what little warm air there is into the flower box.
I have to keep things short and bushy, i have 37 inches to work with, so the SOG thing seems to be the way to go till i can afford to buy materials to build a taller box.
The veg room stays in the low 70s. I'm using a little humidifier in the veg room, keeps it around 50, a little of this is pulled into the flower room, so it stays around 35 in there, i'll update in a week or two and let you know how its going then